What sea creatures use a "cats-eye" to seal the entrance to their shell and what functions does it serve?

Hamish Spencer, a mollusc specialist at the University of Otago's Zoology Department, responded.

The “cat’s-eye” you refer to is a trap-door or operculum used by some sorts of marine snails
to seal the mouth or aperture of their shell when the animal retreats into its shell.
Most marine snails (and some freshwater and land snails) have an operculum.
In the vast majority of species it is not shelly like a cat’s eye, but is instead made
of a more flexible horny or corneous material, often dark brown, and it is sometimes
much smaller than the shell’s aperture.

By contrast, some turban snails (from the family Turbinidae) and nerites (family Neritidae)
have a tight-fitting, solid shelly or calcareous operculum, such as the one often described
as a "cat’s eye". In some tropical turban snails, the operculum is frequently bright green,
blue or orange, and can be 3.5 cm or more across. The common New Zealand
species, Lunella smaragdus, has a blue-green operculum of 10-15 mm,
often found washed up in piles of shell sand.

The main function of corneous opercula in intertidal and land snails is probably to
prevent the animal drying out. Species with calcareous opercula might be
protecting themselves from predators, such as sea stars and carnivorous snails.
If you pick up a living marine snail you will see that its immediate response is
for the animal to retreat into its shell, pulling the operculum behind it. It will
usually emerge after a little time if it is returned to its normal position and
splashed with sea water.

Some tropical conch shell species in the family Strombidae have a modified
dagger-like operculum that the animal can dig into the sand. The animal
then whips its shell and body around, using the operculum as a pivot,
moving forward (or turning over) remarkably quickly
(see http://www.youtube.com/watch?v=YM3sAulcg9M).